Gas used in illuminated signs (4)
I believe the answer is:
neon
'gas used in illuminated signs' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for neon that I've seen before include "Gaseous element, in advertising signs, say" , "Colourless noble gas" , "Element that gives a red glow in a vacuum tube" , "Element 10, used in lights" , "Gaseous element, in advertising sign say" .)